1. Field of the Invention
This invention pertains generally to the field of electrical magnets having support structure.
2. Description of the Prior Art
Energy storage with large superconducting magnets is one possible method of leveling the daily load requirements on present electric generating facilities. The excess energy generated during off-peak hours can be stored in such magnets and later returned to the power system during periods of high loads. Several studies have indicated that augmenting the generating equipment with energy storage devices can be more economical and energy-efficient than simply increasing the generating capacity of the power plants. See, e.g.: R. W. Boom and H. A. Peterson, "Superconductive Energy Storage for Power Systems," IEEE Transactions on Magnetics, Vol. MAG-8, No. 3, Sept. 1972, p. 701.
An energy storage or other large magnet may consist of multiple turns of conductor arranged, for example, in a solenoidal or toroidal configuration. The current flowing in the turns of the magnet produces a net magnetic field. The force on a short element of the conductor in this magnetic field is oriented at right angles to both the current and the magnetic field experienced by the element and is proportional to the products of the magnitudes of the current and the magnetic field. The net component of this force which is in the plane of a conductor turn is directed radially outward in general, and is the major force under consideration here. These forces can be very large in superconducting magnets because of the enormous currents that can be carried by superconductors. For example, if a turn of unsupported conductor in the magnet is substantially circular and planar, the tension at any cross-section in the conductor will be equal to BIR, where B is that component of the magnetic field experienced by the conductor which is perpendicular to the plane of the conductor (axial magnetic field), I is the current in the conductor, and R is the radius of curvature of the turn. The radius of the turns in energy storage magnets under consideration for power systems can be very large, in the range of 10 to 100 meters or more, so that the potential tensile load that an unsupported conductor must carry is very great. Moreover, if a circular conductor begins to yield, the increased radius of the turn causes the tension to increase, which makes the structure inherently unstable. Some external support means is thus necessary to support the conductor. The most commonly accepted support structure for circular turns is a hoop or cylinder of structural material which fits over the circular turn. Magnetically induced forces on the conductor will be opposed by tensile stresses in the conductor and in the external cylinder. This support cylinder must fit very closely over the conductor, since the conductor will experience strain if it must expand to conform to the surrounding cylinder. The cylinder must also resist the radial load with little accompanying strain, since increases in strain in the cylinder will be matched by increases in strain in the conductor in close contact therewith. For very large magnets, this conventional method of support would require massive and uneconomical amounts of structure, in addition to presenting the problem of maintaining close tolerances between the conductor and support structure which must both be of massive dimensions.